If you trace the fuel vent hose from under the tank, back over the carbs to a black plastic thingy clamped on the frame behind the right carb. That thingy is the "tip-over" valve. One of the first things I did when I got my new bike was to remove the valve, then spray dry graphite lube into both ends, and shake it up and down so lube gets to all internal parts (you'll hear the valve inside clicking away when you shake it). Then I re-installed the valve, and it has worked flawlessly. One way to check the valve and vent line is to remove your gas cap, place a length of hose on the metal vent air-entry tube, and try and suck some air, as you would through a drinking straw. If you get lots of air, then your vent line is fine...if it's hard to draw air through, then check to make sure the valve is installed vertically onto it's housing clamp, or re-lube with the graphite. Use a dry lube...Don't use a liquid lube as that will gunk up the valve with time. The bike can be on it's sidestand when you check the air flow.
